This study will investigate brain functioning and language development in autistic, non-ausistic communication-handicapped, and mentally-retarded children. Research has suggested that the language impariment found in autistic children may be related to left hemisphere dysfunction. Also, it has been shown that autistic children typically exhibit an atypical pattern of hemispheric specialization, namely right hemisphere representation for language. This pattern is also found in cases of early left-sided brain trauma but unlike autism, the undamaged right hemisphere is capable of compensating for language. In the proposed research,m the reasons for the failure of right hemisphere compensation will be explored. It wil be determined whether (1) the age at which a child begins language therapy and/or (2) the integrity of right hemisphere functioning predicts an autistic child's level of language attainment. Since plasticity is presumed to decline with age it is predicted that language will be less likely to develop if therapy begins after age 5. In addition, it will be determined whether cerebral asymmetry is a necessary prerequisite for language acquisition in autistic children. By comparing autistic children to those with other forms of communication disorger and mental retardation, it will be determined whether these characteristics are unique to autism. Also, our understanding of these related disorders will be increased. Cerebral lateralization will be measured by the verbal dichotic listening technique. Measures of brain functioning will be simple motor tasks from the Halstead-Reitan Battery. These measures which assess motor weakness can be used to infer lateralizd brain dysfunction. The child will also be administered visuospatial tasks and comprehensive language evaluation. The information gained should allow us to make more educated decisions regarding what therapy should begin to optimize its benefits. More generally, this research seeks to elucidate the parameters and mechanisms underlying plasticity in the humain brain.